[test] Bump shards on slow builder

TBR=nicohartmann@chromium.org

No-Try: true
Change-Id: I4a0e07c3f45868fbfbb233349292e137769f6f51
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2006091Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65830}

infra/testing/builders.pyl

@@ -262,7 +262,7 @@
       {'name': 'mozilla'},
       {'name': 'optimize_for_size'},
       {'name': 'test262', 'variant': 'default'},
-      {'name': 'v8testing', 'shards': 2},
+      {'name': 'v8testing', 'shards': 3},
     ],
   },
   'v8_linux64_dbg_ng_triggered': {
@@ -875,7 +875,7 @@
       {'name': 'mozilla'},
       {'name': 'optimize_for_size'},
       {'name': 'test262', 'variant': 'default'},
-      {'name': 'v8testing'},
+      {'name': 'v8testing', 'shards': 2},
     ],
   },
   'V8 Linux64 - custom snapshot - debug': {